The present invention relates to a method of preparing highly reactive nickel catalysts with an increased lifespan that is used in preparing a synthetic gas by reforming methane, a main component in natural gas, with carbon dioxide. More particularly, the present invention relates to a method of preparing nickel-alumina hybrid aerogel catalyst by using a sol-gel method or by having alumina aerogel to carry nickel thereon, in a different manner than that of the prior nickel catalyst supported on alumina.
Generally, an aerogel is prepared by supercritically drying a gel from a sol-gel reaction. The aerogel has a specific high surface area ratio with a low density. Due to its physical characteristics, aerogel is useful as a catalyst itself or can be used as a carrier for a catalyst. Supercritical drying is a process of drying the gel above a critical state such as high temperature and high pressure to remove solvents held in the gel structures and by this method, it is possible to prevent shrinkage and crack formation due to capillary pressure differences between the gas/liquid phase interface.
Carbon dioxide is considered to be relatively harmless. However, it has been recently revealed as a major factor in global warming, and now its production is restricted by climate control regulation. Therefore, people are now more interested in reducing carbon dioxide production or in developing physical and chemical methods to fix carbon dioxide.
One of the chemical methods in fixing the carbon dioxide is to produce a mixture containing carbon monoxide and hydrogen by reforming methane, a major component in natural gas, with carbon dioxide as shown below. The produced mixture can be utilized in generating a variety of chemicals. This is one of the more effective ways of utilizing the natural gas, which is used mainly as a fuel. It is practically useful to use methane and carbon dioxide simultaneously, considering that methane has a higher global-warming tendency than carbon dioxide and when it is inconvenient to transport the methane, it is wasted by burning it in-situ, which produces undesirable carbon dioxide. EQU CH.sub.4 +CO.sub.2.fwdarw.2CO+2H.sub.2 (.DELTA.H=261 kJ/mol)
The synthetic gas produced by the reaction above has a carbon monoxide/hydrogen ratio of 1:1 and can be used as a starting material for many chemicals. The purity of the produced carbon monoxide is known to be higher than by any other methods. And this reaction characteristic is similar to the conversion method using steam in producing hydrogen gas, and therefore already established processes and knowledge in the field can be utilized. A durable catalyst is urgently needed to be developed since catalyst can be easily deactivated due to the coking during the reaction when the commercial nickel catalyst for the steam reforming is used.
According to the literature, the deactivation of the noble metal catalyst is markedly slow [A. T. Ashcroft, A. K. Cheetham, M. L. H. Green, and P. D. F. Vernon, Nature, 225, 352 (1991)]. Considering the economical aspect, however, it is impractical to use the noble metal catalysts when a large amount of catalyst is needed. Therefore, there have been many studies to improve the durability of the nickel catalyst by using a co-catalyst or additives.
In the commercialized SPARG process, nickel catalyst treated with sulfur is used to improve the durability. In this process, however, there are many problems such as the sulfur must be supplied continuously to the reaction mixture and the reaction needs to be carried out at a temperature higher than 900.degree. C. to improve the reactivity of the catalyst [H. C. Dibbern, P. Olesen, J. R. Rostrup-Nielsen, P. B. Tottrup, and N. R. Udengaard, Hydrocarbon Process., 65, 71 (1986)]. The deactivation of the catalyst can be reduced by adding vanadium or molybdenum into the nickel catalyst [T. Arakawa and M. Oka, U.S. Pat. No. 3,849,087], by adding a basic material such as calcium oxide [Z. L. Zhang and X. E. Verykios, Catal. Today 21, 589 (1994)] or by using basic magnesium oxide that can contain the catalysts.